I think the auxiliary drive belt loses flexibility with ageing rather than with mileage - I have no evidence for this, just a supposition based on the time they seem to last on Morgans.
Agree with Paul, my 4/4 is a 2012 with 13,000 miles but I've changed it on the basis £11.00 for a genuine OE Gates belt and 10 mins to fit it is a small price to pay for peace of mind. I've read stories of Morgan owners having been left stranded at the roadside and even had to be trailored back from France when the belt failed.
